Effects Of MCD And Humic Acid On Aquatic Toxicity And Bioavailability Of The Chiral Herbicide Diclofop Methyl

Widespread application of chiral pesticides results in their occurrence in adjacent environmenat systems such as atmosphere, soil, and surface water. In general, the enantiomers of chiral pesticides are different in some biological processes and thus pose enantioselective ecotoxicity to nontarget biology. Consequently, use and environmental safety of chiral pesticides have been a new point of scientific researches. In the present study, the enantiomers of diclofop methyl (DM) and diclofop (DC) prepared by ourselves were selected to investigate their enantioselective degradation and ecotoxicology in algal cell cultures.DM is rapidly adsorbed to algal cells and is catalytically hydrolyzed to less toxic DC which shows phytotoxicicty. They further metabolize to more toxic phenol metabolites, indicating an increase of environmental burden on DM degradation. DM and its transformation products have different toxicity to algae, associated with their physical chemical properties. Assessing the ecotoxicology and environmental fate of pesticide transformation products is necessary to accurately evaluate environmental risk of pesticides. Toxicology and degradation of chiral herbicides DM and DC is enantioselective, dependent of algae and chemicals. Chlorella pyrenoidosa preferentially uptakes or degrades (R)-enantiomers of DM and DC, however, Chlorella vulgaris and Scenedesmus obliquus have reverse enantioselelctivity with (S)-enantiomer degrading more rapidly than the R enantiomer. The enantioslelctivity of three algae is correlated with their respective cell permeability and is mainly ascribed to cell facilitated transport.Phase-solubility experiments and other methods or technologies demonstrate inclusion complexes between cyclodextrins (CDs) and DM/DC. The inclusion complexation of partially methylated cyclodextrin (MCD) enhances water solubility and stability of DM, improves solution rate of DM, and decreases adsorption of DM to soils, indicating an increase in pesticide bioavailability. MCD does not inhibit algal growth, and decreases passive uptake of algae and cell surface hydrophobicity, but reduces degradation rate of DM and DC in algal cell suspension solutions. The resultsshow that less pesticide have the same biological acitivity when MCD exists. Besides, MCD changes enantioselective degradation of chiral herbicides in alga cultures. Thus, MCD probably increases herbicidal acitivity of DM and DC as additives, reduces application rates of pesticides, and decrease ecological risk of pesticides caused by overdose.Humic acid (HA) is a major component of dissolved organic matter in natural aquatic systems. HA binds DM and DC, and reduces the free pesticide compounds. In the study, however, HA, adsorbed to algal cells, increases cell permeability of three algae, decreases hydrophobicity of algal cell surface, and changes growth character of three algae in different degrees. Data for degradation of DM and DC and algal biomass in Chlorella vulgaris cultures shows that HA increases degradation or uptake of these pesticides and enhance their bioavailability. These suggest that cell permeability induced by HA plays more important role in their bioavailability than the binding interaction between HA and pesticides. At the same time, HA change degradation of DC and its two enantiomers, dependent of HA concentration, algae, and types and concentration of pestcicide.DC at low dose or concentration stimulates the growth of algae. On the algae exposed to 0.10 mg L"1 of DCs enantiomers for two months, however, the induction by DC results in some changes of algal growth and cell permeability. Induction by two enantiomers of DC stimulates the growth of Chlorella pyrenoidosa, inhibits the growth of Chlorella vulgaris, and have no observable effect on Scenedesmus obliquus. In general, induction increases the uptake of the same substrate by algal cells, for example, algal cells induced by R-DC uptake more R-DC than CK. Moreover, induction by two enantiomers reduces or even reverses the enantioselective degradation of chiral pesticides in alga cultures.